Process for preparation of carbon monoxide



`Ial'l. 19, 1960 H, a JOHNSON ETAL 2,921,840

PROCESS FOR PREPARATION OF CARBON MONOXIDE Filed Nov. 9, 1956 G13 /NL frUnited States Patent O PROCESS FOR PREPARATION F CARBON MONOHDE HerbertS. Johnson, Shawinigan Falls, Quebec, and Arthur H. Andersen, MountRoyal, Quebec, Canada, assignors to Shawinigan Chemicals Limited,Montreal, Quebec, Canada, a corporation of Canada Application November9, 1956, Serial No. 621,470

1 Claim. (Cl. 23-204) This invention relates to the preparation ofcarbon monoxide by reaction of carbon dioxide with carbon.

The chemical reaction involved has been the subject of manyinvestigations, a recent one being reported by S. Ergun in I. Phys.Chem. 60, 480 (1956). Ergun passed carbon dioxide upwardly through a bedof carbon particles in a reaction tube to form a lluidized bed ofcarbon, and supplied the endothermic heat of reaction by surrounding thefluidized bed with an electric furnace which heated the uidized bedthrough the wall of the tube. Stalhed et al., in U.S.P. 2,607,667,described the process in which they passed carbon dioxide through aslowly descending packed bed of carbon and supplied the endothermic heatof reaction by passing electric current through the carbon betweenelectrodes penetrating the bed.

The externally heated uidized bed process is highly inefficient inrequiring heat to be transferred through a wall to the reaction, and thepacked bed process is ineflicient due to gas channeling, electriccurrent channeling, and other difficulties (Stalhed, Stahl and Eisen,page 459 (1954)). Y Y

It is an object of this invention to provide a liuidized bed process forreacting carbon dioxide with carbon to form carbon monoxide, in whichprocess the endothermic heat of reaction is generated in the uidizedbed.

It has been observed that it is possible to conduct an electric currentthrough a fluidized bed of carbon particles and to generate sut'cientheat in the uidized bed by the passage of the electric current to supplyendothermic heat of reaction and also, if desired, to raise thetemperature of the bed to a very high degree, for example to red heat.This invention comprises a process for the preparation of carbonmonoxide comprising passing a stream of carbon dioxide gas upwardlythrough a bed comprising finely divided electrically conductive carbonparticles, maintaining the particles of the bed in a fluidized state bythe passage of the said gas and the products of its reaction with thecarbon upwardly therethrough, passing an electric current through theresulting fluidized bed with suflicient power to maintain it at atemperature which sustains a reaction between the carbon dioxide and thecarbon particles, and recovering car- Y bon monoxide coming off theiiuidized bed. The carbon monoxide coming off the fluidized bed can berecovered by any suitable method or means known in the art, and can beutilized, separated, or stored as required.

The accompanying drawing is a diagrammatic sketch showing, partly incross-section, a typical simple apparatus suitable for carrying out theprocess of the invention.

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rent through the uidized bed are shown partly immersed in the bed. A.C.voltage is applied to the electrodes by transformer 6, and gases comingolir the fluidized bed leave the chamber through outlet 7.Alternatively, power can be supplied as direct current.

The chemical reaction taking place in the process of the invention is anequilibrium reaction, and the equilibrium is shifted in the direction ofincreasing conversion of carbon dioxide to carbon monoxide by increasingreaction temperatures. Hence it is desirable to carry out the process ofthe invention at a temperature which provides the optimum economicbalance between conversion and power consumption. Some reaction canoccur at temperatures as low as 600 C. and essentially completeconversion of carbon dioxide to carbon monoxide can theoretically occurabove about 1200 C.

An important factor affecting the completeness of conversion of carbondioxide to carbon monoxide in the process of this invention is the timeof contact of the carbon dioxide with carbon particles -in theuidizedbed. Reaction can occur only during the period of contact in the bed,i.e. while the gases are in the bed. At 800 C., contact times as low as0.5 second provide some conversion, but contact times greater than lsecond are preferred to obtain conversions over 50%. At appropriatetemperatures, high conversions of almost can be Vobtained with contacttimes nov greater than 4 seconds, 'for example slightly over 3 seconds.Contact times longer than that required to reach equilibrium conditionsare unnecessary of course. Contact times can be varied and adjusted byvarying the rate of ow of carbon dioxide into the lluidized bed ofcarbon particles, and by varying the depth of the bed of carbonparticles. Increasing the rate of owof carbon dioxide will decrease thecontact time inV any given bed, and increasing the depth of the bed willincrease the contact time of gas passing at any specific linear flowrate.

Another factor aecting the completeness of conversion of carbon dioxideunder any particular conditions outlined above is the use of catalysts.There are a number of known catalysts for the chemical reaction takingplace, for example alkali hydroxides and alkali carbonates, and thesecatalysts are effective in increasing the rate of reaction inthe'present invention. The catalyst is conveniently utilized byimpregnatiug the carbon particles of the ytluidized bed with it, forexample by use of an aqueous solution thereof.

The invention is illustrated by the following examples of specificlaboratory scale embodiments of it. The examples were carried out in aVycor high temperature resistant glass reactor l2 inches long and about11/2 inches diameter, fitted with a removable bottom made of castablerefractory, through which an inlet opening permitted introduction ofgas. The top of the reactor was covered with a refractory disc fittedwith holes for pas- -sage of a thermocouple, two electrodes, and a gasoutlet. The lowerpart of the reactor was charged with about 10 grams ofcoarse coke particles of about l to 3 millimeters diameter, which actedas a gas distributor to distribute the uidizing gas evenly under theuidized bed. The carbon for the reaction was placed on top of the coarsecoke. In these examples the carbon used was a iluid petroleum coke,produced in a fluid bed petroleum coking process, and calcined at about900 C. for a few seconds to increase its electrical conductivity.(Without calcination, the conductivity of iluid petroleum coke is toolow to permit significant resistive heating of the coke by conduction ofan electric current with the application of voltage gradients as high as1000 volts 'per inch.) The carbon particles were between about 0.3 and 1millimeter diameter in size, i.e. they al1 passed through a screenhaving 16 meshes per linear inch (U.S. Standard Sieve No. 16). 'Ihecarbon dioxide gas for the reaction was passed through a ow. meter(rotameter) and admitted Ito the bottom ofthe reactor; Two4 .graphiteelectrodes were adjustably mounted in the top of the .reactor .servedresults of which are reported in Table I below,

carbon dioxide was passed through the owmeter and into the reactor Yatthe desired rate Vindicated for each example in Table I, measured incubic centimeters of gas per minute at atmospheric pressure and roomtemperature which was about "25 C. The weight in grams of line carboncharged tothe reactor for each example was measured. 'For some oftheexamples the carbon was modied by treatment as follows, before being`charged to the reactor:

('1) YFor Example 7, the carbon was fluidized in a stream Yof air andmaintained at a temperature of 325 C. `for four hours. This treatment,`which can be Ytermed a low temperature preoxidation, converted part ofthe coke to its ultimate combustion products carbon dioxide 'andwatenand increasedthe surface area ofthe remaining lcoke. Complete andalternative details 'fo'rrcarrying out :this Atreatment by lowtemperature preoxidation are given fin U.S.P. 2,721,169.

(2) YFor 'Example 8, `the carbon particles were stirred overnight in200ml. of 3 normal aqueous NaOH solution, :separated from the solutionby filtration, then dried at 110 C. -for'4 hours. This treatmentimpregnated the carbon with about 2-3% of its weight rof NaOH.

-(3) For yExample 9, the 'carbon was preoxidizedas described intreatment (1) above, then impregnated with NaOH as described intreatment (2) above. The carbon fthen'contained about 3-4% NaOHby'wei'ght.

(4) For Example 10, the vcarbon was preoxidized as *described Vintreatment (l) above, then irnpregnated'with VNaOH as described intreatment (2) above except that -the NaOH solution Vused to impregnatethe carbon lwas Vl normal Vinstead 4of 3 normal. The carbon thencontainedabout 2-3 NaOH by weight. Y f VIn -each-of the examples, thecarbon dioxide gas ow Was first adjusted to the desired rate, thenvoltage was applied across the electrodes to force a current through:the bed. The voltage was adjusted to 'give the current value and .powerinput which would maintain the bed at he vtemperature desired, asindicated for each example fin-Table l. `In these examples the voltagesusedvaried between .50 andV 150 volts,and the currents required to-maintainY the desired temperatures in the bed varied be- .tween 7 andl5 amperes. When the desired temperatures -had been reached lin lthe bedand maintained `for 30 minutes, samples of the gases coming from thereactor rwere taken and analyzed with an Orsat apparatus. Thepercentages of CO Vand CO2 in the gases obtained Vin the `variousexamples are shown in Table I, together with the loperatingrdetailspreviously mentioned. The Calculated Contact Time is the time of contactbetween gas' and carbon in the uidized bed in seconds, calculated on an"average of the volumes of gases passing through the bed,

taking into account the following factors and assumptions: (1) Theapparent -density of the carbon bed before 'uidization is about 1, i.e.40 grams of carbon in the charge voccupy a volume of about 40 cc.

(2) The'real density of thecarbon particles is Vabout (3) The proportionof voids in the carbon bed before "uidization, in accordance with theforegoing factors, is about 50%.

(4) The volume of the Yiiuidized bed is about 10% greater than thevolume ofxthe bed of carbon particles before uidization, i.e. the :.gasvolume in the fluidized bed is about 60% of the volume of the carbon bedbefore iluidization.

(5) Gas flowing into the bottom of the uidized bed is assumed tto vbesubstantially vinstantaneously raised to the temperature ofthe fluidizedrbed, increasing Vits volume substantially instantaneously inproportionto the temperature y'change in accordance 'with Gay-Lussacslaw; 'temperatureIdoes not cause Vany other volume change in thegasowingthrough the fluidized bed.

(6) The 'volume of hot gas flowing out the top of the fluidized bed isgreater than the volume of hot gas dowing at the bottom of the fluidizedbed by a factor directly proportional to the conversion ofY carbondioxide to carbon monoxide which occurs in the bed; for example if ofthe carbon dioxide owing into the bed is converted to'carbon monoxide inthe bed, the volume .of .hot gas ilowing out the top of the fluidizedbed will be 80% greater than the volume of hot gas flowing at the bottomof the fluidized bed.

(7) The calculated contact time is an average contact time based on avolumetric rate of ow of gas in the bed which is an average of thevolumetric rates of flow of hot gas at Vthe bottom of the bedand hot gasout the top of the bed.

Table I Y Y Product Gas Carbon Calculated Bed Analysis Ex. No; ChargeCO2 Feed Contact Temp.,

(gm.) (ce/min.) Time C.

. (Sec.) Percent Percent O0 CO2 -40 180 1. 7 800 66. 3 33. 7 40 720 0. 5800 16. 0 84. 0 40 v18() l. 5 1, 000 8l. 1 18. 9 40 '180 1.1 1,200 94. 65. 4 60 180 l. 6 1, 200 96. 0 4. 0 60 A70 3. 2 1, 200 98. 2 1. S 45180 1. 6 800 59. 6 40. 4 v 51 iso 1. 7 soo 7s. 3 21. 7 42. 5 180 l. 4800 92. 0 8. 0 33V 180 Y 1. l 800 79. 0 21.0

V`.From the foregoing disclosure it can be seen that the inventionprovides anV eicient process for the conversion of carbon .dioxide intocarbon monoxide, by which conversions greater than '90% can readily beachieved. The

jprocess 'combines numerous Vadvantages obtained by the use ofaffiuidized bed with numerousadvantages obtained 'by 'the use fo'f 'aninternally heated electrically conductive carbon bed. The ypreferredreaction temperature kfor carrying out the process is around l200 C.,and ex cellent "results are obtained with all temperatures over '800 C.The preferred reaction time for carrying out 'the process provides Vaperiod of contact between reactants of 'about 3seconds, and satisfactoryresults are obtained 'with 'contact periods above about 0.5 seconds. Inpreferred embodiments 'the process utilizes the conventional alkalicatalysts vfor the `reaction and conventional steps Vprocess, andcalcined at about v900 C. for a few seconds to increase .theirlelectrical conductivity, said Vcoke particles having beenimpregnatedwith a catalyst .selected vfrom .the group consisting-of .alkali.hydroxide and alkali Learbonate before being uidized with the carbondioxide and having been treated by low temperature oxidation beforebeing impregnated with said catalyst, maintaining the particles of thebed in a uidized state by the passage of the said gas and the productsof its reaction with the carbon upwardly therethrough, the contact timebetween the carbon particles and the gas passing through the bed beinggreater than 3 seconds and not greater than 4 seconds, passing anelectric current through the resulting uidized bed with suicient powerto maintain it at a temperature between 1000 and 1200n C., andrecovering carbon monoxide coming olf the uidzed bed.

References Cited in the tile of this patent UNTED STATES PATENTS1,857,799 Winkler May 10, 1932 2,652,319 Sweetser et a1 Sept. 15, 1953FOREIGN PATENTS 582,055 Great Britain e Nov. 4, 1946 OTHER REFERENCESHaslam et al.: Fuels and Their Combustion, 1926, pp. 152-158.

